1. Field of the Invention
The present invention relates to a ferroelectric thin film element, and more particularly to an element in which a spontaneous polarization of a ferroelectric thin film is involved in an improvement in device characteristics, such as a non-volatile memory. The present invention also relates to a piezoelectric actuator utilizing a piezoelectric property of an epitaxial film and a liquid discharge head equipped with a piezoelectric actuator unit of a configuration including such a piezoelectric actuator.
2. Description of the Related Art
As a memory medium for a non-volatile memory and the like, there is recently desired a memory apparatus employing a ferroelectric thin film having a high performance (hereinafter called ferroelectric memory). For securing optimal device characteristics and reproducibility in the ferroelectric memory, the ferroelectric thin film is required to have a large spontaneous polarization (residual polarization), a small temperature dependence of the residual polarization, and a small deterioration of inversion of polarization in repeated cycles.
As the ferroelectric material, lead zirconate titanate [Pb(Zr, Ti)O3] (also represented as PZT) is principally employed. PZT is a solid solution of lead zirconate and lead titanate, and a solid solution of a molar ratio of about 1:1 is considered to have a large spontaneous polarization, capable of inversion even under a weak electric field and excellent as a memory medium. As PZT has a relatively high transition temperature (Curie temperature) between a ferroelectric phase and a paraelectric phase of about 300° C. or higher, there is little concern that the memorized content is lost by heat in a temperature range in which ordinary electronic circuits are used (120° C. or lower).
However, even with an excellent ferroelectric thin film such as a PZT thin film, it is difficult to obtain satisfactory device characteristics in a ferroelectric thin film formed by a polycrystalline member, because of a distortion of physical properties at the crystal grain boundary. Therefore, in consideration of the device characteristics of the ferroelectric element, there is desired an epitaxial thin film as close to a complete single crystal as possible.
Also for integration of the ferroelectric devices, a thinner film formation of the ferroelectric thin film is effective, but in case a film thickness of the ferroelectric thin film becomes equal to or less than 100 nm, the spontaneous polarization of the ferroelectric thin film tends to be lost even if it is an epitaxial film, and a deterioration in the residual polarization or the fatigue resistance of the ferroelectric thin film becomes conspicuous. For this reason, in order to reduce the thickness of the ferroelectric thin film, certain measures are required for maintaining the spontaneous polarization of the ferroelectric thin film at a sufficiently high level.
In order to increase the spontaneous polarization of the ferroelectric thin film, there can be employed a method of utilizing a mismatch in the thermal expansion coefficient between the substrate and the ferroelectric thin film (Japanese Patent Application Laid-Open No. H08-186182), or a method of utilizing a misfit in the lattices of the substrate and the ferroelectric thin film (Japanese Patent Application Laid-Open No. H08-139292). These methods can cause a compression stress to be applied to the ferroelectric thin film, thereby increasing the spontaneous polarization of the ferroelectric thin film.
However the prior method of increasing the spontaneous polarization by applying a compression stress to the ferroelectric thin film, though capable of increasing the spontaneous polarization, cannot improve the deterioration in the residual polarization or the fatigue resistance of the ferroelectric thin film. It is estimated that a stress applied in the ferroelectric thin film along a planar direction of the substrate is significantly involved in the aforementioned deterioration of the characteristics of the ferroelectric thin film, and, in case a large compression stress is applied to the ferroelectric thin film in the prior method, the stress applied along the planar direction of the substrate acts on the ferroelectric thin film, thereby further increasing the deterioration in the characteristics of the ferroelectric thin film.
On the other hand, a printer utilizing an ink jet recording apparatus is widely popular as a printing apparatus for a personal computer or the like, because of satisfactory printing ability, simple handling and low cost. An ink jet head employed in such ink jet recording apparatus is a liquid discharge head for discharging ink, and is available in various types such as one generating a bubble in the ink by thermal energy and discharging an ink droplet by a pressure wave caused by such bubble, one suctioning and discharging an ink droplet by an electrostatic force, and one suctioning and discharging an ink droplet utilizing a pressure wave generated by an actuator having a vibrator such as a piezoelectric element or an electrostriction element.
A liquid discharge head utilizing a piezoelectric actuator generally has a pressure chamber communicating with a liquid supply chamber, and a liquid discharge port communicating with such pressure chamber, and, in a part of the pressure chamber, there is provided a vibrating plate on which the piezoelectric actuator is adjoined or directly formed. In the liquid discharge head of such configuration, a predetermined voltage is applied to the piezoelectric actuator to cause an extension-contraction motion of the piezoelectric element, thereby inducing a bending vibration and pressurizing the liquid in the pressure chamber to discharge a liquid droplet from the liquid discharge port.
A color ink jet recording apparatus is currently becoming popular, and there is being requested an improvement in the printing performance, particularly for a higher resolution and a higher printing speed. To this end, it is attempted to achieve a high resolution and high-speed printing by a multi-nozzle head structure through a miniaturization of the liquid discharge head for liquid discharge. For miniaturizing the liquid discharge head, it is necessary to reduce the dimensions of the piezoelectric actuator for liquid discharge.
In the liquid discharge head utilizing the piezoelectric actuator, a compact piezoelectric actuator has conventionally prepared by fine working, such as grinding and polishing, of a piezoelectric member prepared by sintering, but it is separately being investigated to develop an ultra-small piezoelectric actuator having a high precision by forming a piezoelectric member as a film and utilizing a fine working technology developed in the semiconductor field. Also, in view of achieving a higher performance, such a piezoelectric film is preferably a film having a single crystal structure or a crystal orientation property, and a heteroepitaxial growing technology is being actively developed.
Also, in the case of employing a ferroelectric member as a piezoelectric member, a large spontaneous polarization is one of the characteristics desired for the ferroelectric member. However, in the case of a film, a reduction in the film thickness of the ferroelectric film tends, even if the ferroelectric film is an epitaxial film, to lose the spontaneous polarization of the ferroelectric film, and certain measures are required for maintaining the spontaneous polarization of the epitaxial ferroelectric film at a sufficiently high level.
In order to increase the spontaneous polarization of the epitaxial ferroelectric film, as stated above, there can be employed a method of utilizing a mismatch in the thermal expansion coefficient between the substrate and the ferroelectric film (Japanese Patent Application Laid-Open No. H08-186182), or a method of utilizing a misfit in the lattices of the substrate and the ferroelectric film (Japanese Patent Application Laid-Open No. H08-139292). These methods can form an epitaxial ferroelectric film to which a compression stress is applied, thereby obtaining an epitaxial ferroelectric film with a high spontaneous polarization.
However, the prior method of forming an epitaxial ferroelectric film involving a compression stress therein to increase the spontaneous polarization thereby improving the piezoelectric property, though capable of increasing the spontaneous polarization, cannot resolve drawbacks such as deterioration in the characteristics of the piezoelectric actuator over the course of repeated uses or destruction of the piezoelectric actuator induced by a leak current at the application of voltage. It is estimated that a stress applied in the ferroelectric thin film along a planar direction of the substrate is significantly involved in the aforementioned deterioration in the characteristics and destruction of the piezoelectric actuator, and, in an epitaxial ferroelectric film prepared by the prior method and subjected to a large compression stress, the stress applied along the planar direction of the substrate acts on the ferroelectric thin film, thereby further increasing the deterioration of the durability characteristics of the piezoelectric actuator.